We utilized the existing quorum sensing system in bacteria to create a signal that propagates through a bacterial population of Rhizobium leguminosarum upon detection of a mycotoxin produced by Fusarium oxysporum.
F. oxysporum produces a mycotoxin known as fusaric acid, and it can be detected by a fusaric acid-inducible promoter (BBa_K1493002), which is composed of the PP_1262 gene and a promoter. The PP_1262 gene inhibits the binding of RNA polymerase to the promoter, subsequently preventing transcription. But fusaric acid enables transcription by blocking the inhibitory activity of the PP_1262 gene. Hence, in the presence of fusaric acid, the LuxI gene (BBa_K4952000) is transcribed and translated to produce 3-oxo-hexanoyl-HSL, an N-acyl-homoserine lactone (AHL). The 3-oxo-hexanoyl-HSL molecule produced is transported to the cell membrane by a peptide signal (BBa_K4952001) and diffuses out of the cell.
When sufficient 3-oxo-hexanoyl-HSL accumulates in the environment, it diffuses back into the R. leguminosarum cells down the concentration gradient. In the bacterial cells, the LuxR protein (BBa_K4952002, BBa_K49520003, BBa_K49520004, and BBa_C0062) is being produced continuously by a constitutive promoter (BBa_J23119). When 3-oxo-hexanoyl-HSL enters the bacterial cells, it binds to the LuxR protein to form a complex that then activates the Lux pR promoter (BBa_R0062). This leads to the transcription and translation of the LuxI gene, which leads to more production of the 3-oxo-hexanoyl-HSL molecule.
This eventually results in the propagation of the cell signal initiated by the detection of F. oxysporum. The next step is to construct a hardware system that is capable of detecting this propagated cell signal that then informs the farmers of the presence of the pathogenic fungus.